From the Guidelines
To rule out myasthenia gravis, the most appropriate initial test is single-fiber electromyography (SFEMG), which is positive in over 90% of patients with ocular myasthenia and is considered the gold standard for diagnosis in many centers. This test is highly sensitive and can help establish the diagnosis by demonstrating the autoimmune attack on neuromuscular junction components that causes the characteristic fatigable muscle weakness seen in myasthenia gravis 1.
Key Points to Consider
- The presence of antiacetylcholine receptor antibody (AChR-Ab-binding, blocking, or modulating) can confirm the diagnosis, but about 20% of patients with generalized myasthenia and about half of those with ocular myasthenia are seronegative 1.
- Repetitive nerve stimulation testing (RNS) may also assist in diagnosis, but it is positive in only one-third of patients with ocular myasthenia 1.
- The edrophonium (Tensilon) test can provide additional diagnostic information, but it is less commonly used today due to potential cardiac risks and limited availability, and should be performed in a monitored setting with atropine available for potential administration 1.
- Other tests, such as anti-muscle-specific kinase (MuSK) antibodies and anti-lipoprotein receptor-related protein 4 (LRP4) antibodies, may also be useful in identifying other forms of the disease, especially in seronegative patients 1.
Diagnostic Approach
The diagnostic approach to myasthenia gravis should involve a combination of clinical evaluation, laboratory tests, and electrophysiological studies.
- A complete sensorimotor examination and external examination is critical, with attention to the presence of strabismus, ptosis, and slow saccades 1.
- The Ice Test, which involves applying an ice pack over the closed eyes for 2 minutes in the case of ptosis and for 5 minutes in the case of strabismus, may demonstrate a reduction of ptosis and misalignment, and is highly specific to this disorder 1.
- A rest test without an ice pack can also be used to aid in diagnosis 1.
- Chest imaging (CT or MRI) should be performed to evaluate for thymoma, as approximately 10-15% of myasthenia gravis patients have associated thymic abnormalities.
From the FDA Drug Label
DOSAGE AND ADMINISTRATION ENLON Test in the Differential Diagnosis of Myasthenia Gravis The ENLON Test for Evaluation of Treatment Requirements in Myasthenia Gravis Responses to ENLON in myasthenic and nonmyasthenic individuals are summarized in the following chart The test used to rule out Myasthenia Gravis (MG) is the ENLON Test.
- The ENLON Test is used in the differential diagnosis of Myasthenia Gravis.
- It involves the administration of edrophonium (IV) to evaluate the response of the patient.
- The response to the test can help establish the diagnosis of Myasthenia Gravis and differentiate it from other conditions. 2
From the Research
Diagnostic Tests for Myasthenia Gravis
The following tests are used to diagnose Myasthenia Gravis (MG):
- Radioimmunoprecipitation assays (RIPA) for detecting acetylcholine receptor (AChR) and muscle-specific kinase (MuSK) antibodies 3, 4, 5, 6
- Cell-Based assays (CBAs) for detecting AChR and MuSK antibodies, which have higher sensitivity than RIPA 3, 4, 7
- Binding and blocking assays for AChR antibodies by RIPA and/or f-CBA, followed by reflex testing for MuSK antibodies if AChR antibodies are negative 4
- Clustered L-CBA for detecting antibodies to low-density lipoprotein receptor-related protein 4 (LRP4) and other autoantigens 4, 6
Sensitivity and Specificity of Diagnostic Tests
The sensitivity and specificity of diagnostic tests for MG vary:
- RIPA has high specificity (nearly 100%) but lower sensitivity (50-92%) for detecting AChR antibodies 4
- CBAs have higher sensitivity than RIPA for detecting AChR and MuSK antibodies 3, 4, 7
- The sensitivity and specificity of RIPA for detecting MuSK antibodies are lower than for AChR antibodies 4
Clinical Applications of Diagnostic Tests
Diagnostic tests are crucial for: